X-Git-Url: http://git.tuebingen.mpg.de/?p=paraslash.git;a=blobdiff_plain;f=crypt.c;h=6f7e611d0364035560819f7659bbb71d62d4f1b1;hp=6d394530ffe19a5ab9294fb6f6021d0bf0490410;hb=bda95f9508b456dcea89d300f6d4104e30ab9f3e;hpb=471684761a2039bbc89aa1e3c33c62de6bef86cf

diff --git a/crypt.c b/crypt.c
index 6d394530..6f7e611d 100644
--- a/crypt.c
+++ b/crypt.c
@@ -1,21 +1,100 @@
 /*
- * Copyright (C) 2005-2007 Andre Noll <maan@systemlinux.org>
+ * Copyright (C) 2005-2011 Andre Noll <maan@systemlinux.org>
  *
  * Licensed under the GPL v2. For licencing details see COPYING.
  */
 
-/** \file crypt.c openssl-based RSA encryption/decryption routines */
+/** \file crypt.c Openssl-based encryption/decryption routines. */
+
+#include <regex.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <openssl/rand.h>
+#include <openssl/err.h>
+#include <openssl/rc4.h>
+#include <openssl/pem.h>
+#include <openssl/sha.h>
+#include <openssl/bn.h>
 
 #include "para.h"
 #include "error.h"
 #include "string.h"
 #include "crypt.h"
+#include "fd.h"
+
+struct asymmetric_key {
+	RSA *rsa;
+};
+
+/**
+ * Fill a buffer with random content.
+ *
+ * \param buf The buffer to fill.
+ * \param num The size of \a buf in bytes.
+ *
+ * This function puts \a num cryptographically strong pseudo-random bytes into
+ * buf. If libssl can not guarantee an unpredictable byte sequence (for example
+ * because the PRNG has not been seeded with enough randomness) the function
+ * logs an error message and calls exit().
+ */
+void get_random_bytes_or_die(unsigned char *buf, int num)
+{
+	unsigned long err;
+
+	/* RAND_bytes() returns 1 on success, 0 otherwise. */
+	if (RAND_bytes(buf, num) == 1)
+		return;
+	err = ERR_get_error();
+	PARA_EMERG_LOG("%s\n", ERR_reason_error_string(err));
+	exit(EXIT_FAILURE);
+}
+
+/**
+ * Seed pseudo random number generators.
+ *
+ * This function reads 64 bytes from /dev/urandom and adds them to the SSL
+ * PRNG. It also seeds the PRNG used by random() with a random seed obtained
+ * from SSL. If /dev/random could not be read, an error message is logged and
+ * the function calls exit().
+ *
+ * \sa RAND_load_file(3), \ref get_random_bytes_or_die(), srandom(3),
+ * random(3), \ref para_random().
+ */
+void init_random_seed_or_die(void)
+{
+	int seed, ret = RAND_load_file("/dev/urandom", 64);
+
+	if (ret != 64) {
+		PARA_EMERG_LOG("could not seed PRNG (ret = %d)\n", ret);
+		exit(EXIT_FAILURE);
+	}
+	get_random_bytes_or_die((unsigned char *)&seed, sizeof(seed));
+	srandom(seed);
+}
+
+static int check_key_file(const char *file, int private)
+{
+	struct stat st;
+
+	if (stat(file, &st) != 0)
+		return -ERRNO_TO_PARA_ERROR(errno);
+	if (private != LOAD_PRIVATE_KEY)
+		return 0;
+	if ((st.st_uid == getuid()) && (st.st_mode & 077) != 0)
+		return -E_KEY_PERM;
+	return 1;
+}
 
 static EVP_PKEY *load_key(const char *file, int private)
 {
 	BIO *key;
 	EVP_PKEY *pkey = NULL;
+	int ret = check_key_file(file, private);
 
+	if (ret < 0) {
+		PARA_ERROR_LOG("%s\n", para_strerror(-ret));
+		return NULL;
+	}
 	key = BIO_new(BIO_s_file());
 	if (!key)
 		return NULL;
@@ -29,18 +108,7 @@ static EVP_PKEY *load_key(const char *file, int private)
 	return pkey;
 }
 
-/**
- * read an RSA key from a file
- *
- * \param key_file the file containing the key
- * \param rsa RSA structure is returned here
- * \param private if non-zero, read the private key, otherwise the public key
- *
- * \return The size of the RSA key on success, negative on errors.
- *
- * \sa openssl(1), rsa(1).
- */
-int get_rsa_key(char *key_file, RSA **rsa, int private)
+static int get_openssl_key(const char *key_file, RSA **rsa, int private)
 {
 	EVP_PKEY *key = load_key(key_file, private);
 
@@ -54,26 +122,358 @@ int get_rsa_key(char *key_file, RSA **rsa, int private)
 	return RSA_size(*rsa);
 }
 
+#define KEY_TYPE_TXT "ssh-rsa"
+
+/* check if it is an ssh rsa key */
+static size_t is_ssh_rsa_key(char *data, size_t size)
+{
+	char *cp;
+
+	if (size < strlen(KEY_TYPE_TXT) + 2)
+		return 0;
+	cp = memchr(data, ' ', size);
+	if (cp == NULL)
+		return 0;
+	if (strncmp(KEY_TYPE_TXT, data, strlen(KEY_TYPE_TXT)))
+		return 0;
+	cp++;
+	if (cp >= data + size)
+		return 0;
+	if (*cp == '\0')
+		return 0;
+	return cp - data;
+}
+
+/*
+ * This base64/uudecode stuff below is taken from openssh-5.2p1, Copyright (c)
+ * 1996 by Internet Software Consortium.  Portions Copyright (c) 1995 by
+ * International Business Machines, Inc.
+ */
+
+static const char Base64[] =
+	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+static const char Pad64 = '=';
+/*
+ * Skips all whitespace anywhere. Converts characters, four at a time, starting
+ * at (or after) src from base - 64 numbers into three 8 bit bytes in the
+ * target area. it returns the number of data bytes stored at the target, or -1
+ * on error.
+ */
+static int base64_decode(char const *src, unsigned char *target, size_t targsize)
+{
+	unsigned int tarindex, state;
+	int ch;
+	char *pos;
+
+	state = 0;
+	tarindex = 0;
+
+	while ((ch = *src++) != '\0') {
+		if (para_isspace(ch)) /* Skip whitespace anywhere. */
+			continue;
+
+		if (ch == Pad64)
+			break;
+
+		pos = strchr(Base64, ch);
+		if (pos == 0) /* A non-base64 character. */
+			return -1;
+
+		switch (state) {
+		case 0:
+			if (target) {
+				if (tarindex >= targsize)
+					return (-1);
+				target[tarindex] = (pos - Base64) << 2;
+			}
+			state = 1;
+			break;
+		case 1:
+			if (target) {
+				if (tarindex + 1 >= targsize)
+					return (-1);
+				target[tarindex]   |=  (pos - Base64) >> 4;
+				target[tarindex+1]  = ((pos - Base64) & 0x0f)
+							<< 4 ;
+			}
+			tarindex++;
+			state = 2;
+			break;
+		case 2:
+			if (target) {
+				if (tarindex + 1 >= targsize)
+					return (-1);
+				target[tarindex]   |=  (pos - Base64) >> 2;
+				target[tarindex+1]  = ((pos - Base64) & 0x03)
+							<< 6;
+			}
+			tarindex++;
+			state = 3;
+			break;
+		case 3:
+			if (target) {
+				if (tarindex >= targsize)
+					return (-1);
+				target[tarindex] |= (pos - Base64);
+			}
+			tarindex++;
+			state = 0;
+			break;
+		}
+	}
+
+	/*
+	 * We are done decoding Base-64 chars.  Let's see if we ended
+	 * on a byte boundary, and/or with erroneous trailing characters.
+	 */
+
+	if (ch == Pad64) {		/* We got a pad char. */
+		ch = *src++;		/* Skip it, get next. */
+		switch (state) {
+		case 0:		/* Invalid = in first position */
+		case 1:		/* Invalid = in second position */
+			return (-1);
+
+		case 2:		/* Valid, means one byte of info */
+			/* Skip any number of spaces. */
+			for (; ch != '\0'; ch = *src++)
+				if (!isspace(ch))
+					break;
+			/* Make sure there is another trailing = sign. */
+			if (ch != Pad64)
+				return (-1);
+			ch = *src++;		/* Skip the = */
+			/* Fall through to "single trailing =" case. */
+			/* FALLTHROUGH */
+
+		case 3:		/* Valid, means two bytes of info */
+			/*
+			 * We know this char is an =.  Is there anything but
+			 * whitespace after it?
+			 */
+			for (; ch != '\0'; ch = *src++)
+				if (!isspace(ch))
+					return (-1);
+
+			/*
+			 * Now make sure for cases 2 and 3 that the "extra"
+			 * bits that slopped past the last full byte were
+			 * zeros.  If we don't check them, they become a
+			 * subliminal channel.
+			 */
+			if (target && target[tarindex] != 0)
+				return (-1);
+		}
+	} else {
+		/*
+		 * We ended by seeing the end of the string.  Make sure we
+		 * have no partial bytes lying around.
+		 */
+		if (state != 0)
+			return (-1);
+	}
+
+	return (tarindex);
+}
+
+static int uudecode(const char *src, unsigned char *target, size_t targsize)
+{
+	int len;
+	char *encoded, *p;
+
+	/* copy the 'readonly' source */
+	encoded = para_strdup(src);
+	/* skip whitespace and data */
+	for (p = encoded; *p == ' ' || *p == '\t'; p++)
+		;
+	for (; *p != '\0' && *p != ' ' && *p != '\t'; p++)
+		;
+	/* and remove trailing whitespace because base64_decode needs this */
+	*p = '\0';
+	len = base64_decode(encoded, target, targsize);
+	free(encoded);
+	return len >= 0? len : -E_BASE64;
+}
+
+/*
+ * The public key loading functions below were inspired by corresponding code
+ * of openssh-5.2p1, Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo,
+ * Finland. However, not much of the original code remains.
+ */
+
+
+/*
+ * Can not use the inline functions of portable_io.h here because the byte
+ * order is different.
+ */
+static uint32_t read_ssh_u32(const void *vp)
+{
+	const unsigned char *p = (const unsigned char *)vp;
+	uint32_t v;
+
+	v  = (uint32_t)p[0] << 24;
+	v |= (uint32_t)p[1] << 16;
+	v |= (uint32_t)p[2] << 8;
+	v |= (uint32_t)p[3];
+
+	return v;
+}
+
+static int read_bignum(const unsigned char *buf, size_t len, BIGNUM **result)
+{
+	const unsigned char *p = buf, *end = buf + len;
+	uint32_t bnsize;
+	BIGNUM *bn;
+
+	if (p + 4 < p)
+		return -E_BIGNUM;
+	if (p + 4 > end)
+		return -E_BIGNUM;
+	bnsize = read_ssh_u32(p);
+	PARA_DEBUG_LOG("bnsize: %u\n", bnsize);
+	p += 4;
+	if (p + bnsize < p)
+		return -E_BIGNUM;
+	if (p + bnsize > end)
+		return -E_BIGNUM;
+	if (bnsize > 8192)
+		return -E_BIGNUM;
+	bn = BN_bin2bn(p, bnsize, NULL);
+	if (!bn)
+		return -E_BIGNUM;
+	*result = bn;
+	return bnsize + 4;
+}
+
+static int read_rsa_bignums(const unsigned char *blob, int blen, RSA **result)
+{
+	int ret;
+	RSA *rsa;
+	const unsigned char *p = blob, *end = blob + blen;
+	uint32_t rlen;
+
+	*result = NULL;
+	if (p + 4 > end)
+		return -E_BIGNUM;
+	rlen = read_ssh_u32(p);
+	p += 4;
+	if (p + rlen < p)
+		return -E_BIGNUM;
+	if (p + rlen > end)
+		return -E_BIGNUM;
+	if (rlen < strlen(KEY_TYPE_TXT))
+		return -E_BIGNUM;
+	PARA_DEBUG_LOG("type: %s, rlen: %d\n", p, rlen);
+	if (strncmp((char *)p, KEY_TYPE_TXT, strlen(KEY_TYPE_TXT)))
+		return -E_BIGNUM;
+	p += rlen;
+
+	rsa = RSA_new();
+	if (!rsa)
+		return -E_BIGNUM;
+	ret = read_bignum(p, end - p, &rsa->e);
+	if (ret < 0)
+		goto fail;
+	p += ret;
+	ret = read_bignum(p, end - p, &rsa->n);
+	if (ret < 0)
+		goto fail;
+	*result = rsa;
+	return 1;
+fail:
+	if (rsa)
+		RSA_free(rsa);
+	return ret;
+}
+
 /**
- * free an RSA structure
+ * Read an asymmetric key from a file.
+ *
+ * \param key_file The file containing the key.
+ * \param private if non-zero, read the private key, otherwise the public key.
+ * \param result The key structure is returned here.
  *
- * \param rsa pointer to the RSA struct to free
+ * \return The size of the key on success, negative on errors.
  *
- * This must be called for any key obtained by get_rsa_key().
+ * \sa openssl(1), rsa(1).
  */
-void rsa_free(RSA *rsa)
+int get_asymmetric_key(const char *key_file, int private,
+		struct asymmetric_key **result)
 {
-	if (rsa)
-		RSA_free(rsa);
+	struct asymmetric_key *key = NULL;
+	void *map = NULL;
+	unsigned char *blob = NULL;
+	size_t map_size, blob_size;
+	int ret, ret2;
+	char *cp;
+
+	key = para_malloc(sizeof(*key));
+	if (private) {
+		ret = get_openssl_key(key_file, &key->rsa, LOAD_PRIVATE_KEY);
+		goto out;
+	}
+	ret = mmap_full_file(key_file, O_RDONLY, &map, &map_size, NULL);
+	if (ret < 0)
+		goto out;
+	ret = is_ssh_rsa_key(map, map_size);
+	if (!ret) {
+		ret = para_munmap(map, map_size);
+		map = NULL;
+		if (ret < 0)
+			goto out;
+		ret = get_openssl_key(key_file, &key->rsa, LOAD_PUBLIC_KEY);
+		goto out;
+	}
+	cp = map + ret;
+	PARA_INFO_LOG("decoding public rsa-ssh key %s\n", key_file);
+	ret = -ERRNO_TO_PARA_ERROR(EOVERFLOW);
+	if (map_size > INT_MAX / 4)
+		goto out;
+	blob_size = 2 * map_size;
+	blob = para_malloc(blob_size);
+	ret = uudecode(cp, blob, blob_size);
+	if (ret < 0)
+		goto out;
+	ret = read_rsa_bignums(blob, ret, &key->rsa);
+	if (ret < 0)
+		goto out;
+	ret = RSA_size(key->rsa);
+out:
+	ret2 = para_munmap(map, map_size);
+	if (ret >= 0 && ret2 < 0)
+		ret = ret2;
+	if (ret < 0) {
+		free(key);
+		result = NULL;
+		PARA_ERROR_LOG("key %s: %s\n", key_file, para_strerror(-ret));
+	} else
+		*result = key;
+	free(blob);
+	return ret;
+}
+
+/**
+ * Deallocate an asymmetric key structure.
+ *
+ * \param key Pointer to the key structure to free.
+ *
+ * This must be called for any key obtained by get_asymmetric_key().
+ */
+void free_asymmetric_key(struct asymmetric_key *key)
+{
+	if (!key)
+		return;
+	RSA_free(key->rsa);
+	free(key);
 }
 
 /**
- * decrypt a buffer using an RSA key
+ * Decrypt a buffer using a private key.
  *
- * \param key_file full path of the rsa key
- * \param outbuf the output buffer
- * \param inbuf the encrypted input buffer
- * \param rsa_inlen the length of \a inbuf
+ * \param key_file Full path of the key.
+ * \param outbuf The output buffer.
+ * \param inbuf The encrypted input buffer.
+ * \param inlen The length of \a inbuf in bytes.
  *
  * The \a outbuf must be large enough to hold at least \a rsa_inlen bytes.
  *
@@ -81,92 +481,225 @@ void rsa_free(RSA *rsa)
  *
  * \sa RSA_private_decrypt(3)
  **/
-int para_decrypt_buffer(char *key_file, unsigned char *outbuf, unsigned char *inbuf,
-			unsigned rsa_inlen)
+int priv_decrypt(const char *key_file, unsigned char *outbuf,
+		unsigned char *inbuf, int inlen)
 {
-	RSA *rsa;
-	int ret, inlen = rsa_inlen;
+	struct asymmetric_key *priv;
+	int ret;
 
 	if (inlen < 0)
 		return -E_RSA;
-	ret = get_rsa_key(key_file, &rsa, LOAD_PRIVATE_KEY);
+	ret = get_asymmetric_key(key_file, LOAD_PRIVATE_KEY, &priv);
 	if (ret < 0)
 		return ret;
-	ret = RSA_private_decrypt(inlen, inbuf, outbuf, rsa, RSA_PKCS1_PADDING);
-	rsa_free(rsa);
-	return (ret > 0)? ret : -E_DECRYPT;
+	/*
+	 * RSA is vulnerable to timing attacks. Generate a random blinding
+	 * factor to protect against this kind of attack.
+	 */
+	ret = -E_BLINDING;
+	if (RSA_blinding_on(priv->rsa, NULL) == 0)
+		goto out;
+	ret = RSA_private_decrypt(inlen, inbuf, outbuf, priv->rsa,
+		RSA_PKCS1_OAEP_PADDING);
+	RSA_blinding_off(priv->rsa);
+	if (ret <= 0)
+		ret = -E_DECRYPT;
+out:
+	free_asymmetric_key(priv);
+	return ret;
+}
+
+/**
+ * Encrypt a buffer using an RSA key
+ *
+ * \param pub: The public key.
+ * \param inbuf The input buffer.
+ * \param len The length of \a inbuf.
+ * \param outbuf The output buffer.
+ *
+ * \return The size of the encrypted data on success, negative on errors.
+ *
+ * \sa RSA_public_encrypt(3)
+ */
+int pub_encrypt(struct asymmetric_key *pub, unsigned char *inbuf,
+		unsigned len, unsigned char *outbuf)
+{
+	int ret, flen = len; /* RSA_public_encrypt expects a signed int */
+
+	if (flen < 0)
+		return -E_ENCRYPT;
+	ret = RSA_public_encrypt(flen, inbuf, outbuf, pub->rsa,
+		RSA_PKCS1_OAEP_PADDING);
+	return ret < 0? -E_ENCRYPT : ret;
+}
+
+struct stream_cipher {
+	RC4_KEY key;
+};
+
+/**
+ * Allocate and initialize a stream cipher structure.
+ *
+ * \param data The key.
+ * \param len The size of the key.
+ *
+ * \return A new stream cipher structure.
+ */
+struct stream_cipher *sc_new(const unsigned char *data, int len)
+{
+	struct stream_cipher *sc = para_malloc(sizeof(*sc));
+	RC4_set_key(&sc->key, len, data);
+	return sc;
 }
 
 /**
- * decrypt the challenge number sent by para_server
+ * Deallocate a stream cipher structure.
  *
- * \param key_file full path of the rsa key
- * \param challenge_nr result is stored here
- * \param inbuf the input buffer
- * \param rsa_inlen the length of \a inbuf
+ * \param sc A stream cipher previously obtained by sc_new().
+ */
+void sc_free(struct stream_cipher *sc)
+{
+	free(sc);
+}
+
+/**
+ * The RC4() implementation of openssl apparently reads and writes data in
+ * blocks of 8 bytes. So we have to make sure our buffer sizes are a multiple
+ * of this.
+ */
+#define RC4_ALIGN 8
+
+/**
+ * Encrypt and send a buffer.
+ *
+ * \param scc The context.
+ * \param buf The buffer to send.
+ * \param len The size of \a buf in bytes.
  *
- * \return positive on success, negative on errors
+ * \return The return value of the underyling call to write_all().
  *
- * \sa para_decrypt_buffer()
+ * \sa \ref write_all(), RC4(3).
  */
-int para_decrypt_challenge(char *key_file, long unsigned *challenge_nr,
-		unsigned char *inbuf, unsigned rsa_inlen)
+int sc_send_bin_buffer(struct stream_cipher_context *scc, const char *buf,
+		size_t len)
 {
-	unsigned char *rsa_out = OPENSSL_malloc(rsa_inlen + 1);
-	int ret = para_decrypt_buffer(key_file, rsa_out, inbuf, rsa_inlen);
+	int ret;
+	unsigned char *tmp;
+	static unsigned char remainder[RC4_ALIGN];
+	size_t l1 = ROUND_DOWN(len, RC4_ALIGN), l2 = ROUND_UP(len, RC4_ALIGN);
 
-	if (ret >= 0) {
-		rsa_out[ret] = '\0';
-		ret = sscanf((char *)rsa_out, "%lu", challenge_nr) == 1?
-			1 : -E_CHALLENGE;
+	assert(len);
+	tmp = para_malloc(l2);
+	RC4(&scc->send->key, l1, (const unsigned char *)buf, tmp);
+	if (len > l1) {
+		memcpy(remainder, buf + l1, len - l1);
+		RC4(&scc->send->key, len - l1, remainder, tmp + l1);
 	}
-	OPENSSL_free(rsa_out);
+	ret = write_all(scc->fd, (char *)tmp, &len);
+	free(tmp);
 	return ret;
 }
 
 /**
- * encrypt a buffer using an RSA key
+ * Encrypt and send a \p NULL-terminated buffer.
  *
- * \param rsa: public rsa key
- * \param inbuf the input buffer
- * \param len the length of \a inbuf
- * \param outbuf the output buffer
+ * \param scc The context.
+ * \param buf The buffer to send.
+ *
+ * \return The return value of the underyling call to sc_send_bin_buffer().
+ */
+int sc_send_buffer(struct stream_cipher_context *scc, const char *buf)
+{
+	return sc_send_bin_buffer(scc, buf, strlen(buf));
+}
+
+/**
+ * Format, encrypt and send a buffer.
  *
- * \return The size of the encrypted data on success, negative on errors
+ * \param scc The context.
+ * \param fmt A format string.
  *
- * \sa RSA_public_encrypt(3)
+ * \return The return value of the underyling call to sc_send_buffer().
  */
-int para_encrypt_buffer(RSA *rsa, unsigned char *inbuf,
-		unsigned len, unsigned char *outbuf)
+__printf_2_3 int sc_send_va_buffer(struct stream_cipher_context *scc,
+		const char *fmt, ...)
 {
-	int ret, flen = len; /* RSA_public_encrypt expects a signed int */
+	char *msg;
+	int ret;
 
-	if (flen < 0)
-		return -E_ENCRYPT;
-	ret = RSA_public_encrypt(flen, inbuf, outbuf, rsa, RSA_PKCS1_PADDING);
-	return ret < 0?  -E_ENCRYPT : ret;
+	PARA_VSPRINTF(fmt, msg);
+	ret = sc_send_buffer(scc, msg);
+	free(msg);
+	return ret;
 }
 
 /**
- * encrypt the given challenge number
+ * Receive a buffer and decrypt it.
  *
- * \param rsa: public rsa key
- * \param challenge_nr the number to be encrypted
- * \param outbuf the output buffer
+ * \param scc The context.
+ * \param buf The buffer to write the decrypted data to.
+ * \param size The size of \a buf.
  *
- * \a outbuf must be at least 64 bytes long
+ * \return The number of bytes received on success, negative on errors, zero if
+ * the peer has performed an orderly shutdown.
  *
- * \return The size of the encrypted data on success, negative on errors
+ * \sa recv(2), RC4(3).
+ */
+int sc_recv_bin_buffer(struct stream_cipher_context *scc, char *buf,
+		size_t size)
+{
+	unsigned char *tmp = para_malloc(size);
+	ssize_t ret = recv(scc->fd, tmp, size, 0);
+
+	if (ret > 0)
+		RC4(&scc->recv->key, ret, tmp, (unsigned char *)buf);
+	else if (ret < 0)
+		ret = -ERRNO_TO_PARA_ERROR(errno);
+	free(tmp);
+	return ret;
+}
+
+/**
+ * Receive a buffer, decrypt it and write terminating NULL byte.
+ *
+ * \param scc The context.
+ * \param buf The buffer to write the decrypted data to.
+ * \param size The size of \a buf.
  *
- * \sa para_encrypt_buffer()
+ * Read at most \a size - 1 bytes from file descriptor given by \a scc, decrypt
+ * the received data and write a NULL byte at the end of the decrypted data.
  *
+ * \return The return value of the underlying call to \ref
+ * sc_recv_bin_buffer().
  */
-int para_encrypt_challenge(RSA* rsa, long unsigned challenge_nr,
-	unsigned char *outbuf)
+int sc_recv_buffer(struct stream_cipher_context *scc, char *buf, size_t size)
 {
-	unsigned char *inbuf = (unsigned char*) make_message("%lu", challenge_nr);
-	int ret = para_encrypt_buffer(rsa, inbuf, strlen((char *)inbuf), outbuf);
-	free(inbuf);
-	return ret;
+	int n;
+
+	assert(size);
+	n = sc_recv_bin_buffer(scc, buf, size - 1);
+	if (n >= 0)
+		buf[n] = '\0';
+	else
+		*buf = '\0';
+	return n;
 }
 
+/**
+ * Compute the hash of the given input data.
+ *
+ * \param data Pointer to the data to compute the hash value from.
+ * \param len The length of \a data in bytes.
+ * \param hash Result pointer.
+ *
+ * \a hash must point to an area at least \p HASH_SIZE bytes large.
+ *
+ * \sa sha(3), openssl(1).
+ * */
+void hash_function(const char *data, unsigned long len, unsigned char *hash)
+{
+	SHA_CTX c;
+	SHA1_Init(&c);
+	SHA1_Update(&c, data, len);
+	SHA1_Final(hash, &c);
+}