Fix alignment of results of RSA operations when using libgcrypt.

If the result of an RSA encryption or decryption operation can be represented
in less bytes than given, gcry_mpi_print() will not add leading zero bytes. Fix
this by adding those ourself.

diff --git a/src/gcrypt/rsa.c b/src/gcrypt/rsa.c
index 5e4ba41..a729591 100644 (file)
--- a/src/gcrypt/rsa.c
+++ b/src/gcrypt/rsa.c
@@ -276,6 +276,10 @@ bool rsa_public_encrypt(rsa_t *rsa, void *in, size_t len, void *out) {
        gcry_mpi_t outmpi = gcry_mpi_new(len * 8);
        gcry_mpi_powm(outmpi, inmpi, rsa->e, rsa->n);
 
+       int pad = len - (gcry_mpi_get_nbits(outmpi) + 7) / 8;
+       while(pad--)
+               *(char *)out++ = 0;
+
        check(gcry_mpi_print(GCRYMPI_FMT_USG, out,len, NULL, outmpi));
 
        return true;
@@ -288,6 +292,10 @@ bool rsa_private_decrypt(rsa_t *rsa, void *in, size_t len, void *out) {
        gcry_mpi_t outmpi = gcry_mpi_new(len * 8);
        gcry_mpi_powm(outmpi, inmpi, rsa->d, rsa->n);
 
+       int pad = len - (gcry_mpi_get_nbits(outmpi) + 7) / 8;
+       while(pad--)
+               *(char *)out++ = 0;
+
        check(gcry_mpi_print(GCRYMPI_FMT_USG, out,len, NULL, outmpi));
 
        return true;