SP 800-56B steps enumerated.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8770)

1 files changed, 27 insertions, 0 deletions

diff --git a/crypto/rsa/rsa_oaep.c b/crypto/rsa/rsa_oaep.c
index 9affabb99d..0945d4f60d 100644
--- a/crypto/rsa/rsa_oaep.c
+++ b/crypto/rsa/rsa_oaep.c
@@ -38,6 +38,13 @@ int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
                                            param, plen, NULL, NULL);
 }
 
+/*
+ * Perform ihe padding as per NIST 800-56B 7.2.2.3
+ *      from (K) is the key material.
+ *      param (A) is the additional input.
+ * Step numbers are included here but not in the constant time inverse below
+ * to avoid complicating an already difficult enough function.
+ */
 int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
                                     const unsigned char *from, int flen,
                                     const unsigned char *param, int plen,
@@ -57,6 +64,7 @@ int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
 
     mdlen = EVP_MD_size(md);
 
+    /* step 2b: check KLen > nLen - 2 HLen - 2 */
     if (flen > emlen - 2 * mdlen - 1) {
         RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP_MGF1,
                RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
@@ -69,15 +77,20 @@ int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
         return 0;
     }
 
+    /* step 3i: EM = 00000000 || maskedMGF || maskedDB */
     to[0] = 0;
     seed = to + 1;
     db = to + mdlen + 1;
 
+    /* step 3a: hash the additional input */
     if (!EVP_Digest((void *)param, plen, db, NULL, md, NULL))
         goto err;
+    /* step 3b: zero bytes array of length nLen - KLen - 2 HLen -2 */
     memset(db + mdlen, 0, emlen - flen - 2 * mdlen - 1);
+    /* step 3c: DB = HA || PS || 00000001 || K */
     db[emlen - flen - mdlen - 1] = 0x01;
     memcpy(db + emlen - flen - mdlen, from, (unsigned int)flen);
+    /* step 3d: generate random byte string */
     if (RAND_bytes(seed, mdlen) <= 0)
         goto err;
 
@@ -88,13 +101,17 @@ int RSA_padding_add_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
         goto err;
     }
 
+    /* step 3e: dbMask = MGF(mgfSeed, nLen - HLen - 1) */
     if (PKCS1_MGF1(dbmask, dbmask_len, seed, mdlen, mgf1md) < 0)
         goto err;
+    /* step 3f: maskedDB = DB XOR dbMask */
     for (i = 0; i < dbmask_len; i++)
         db[i] ^= dbmask[i];
 
+    /* step 3g: mgfSeed = MGF(maskedDB, HLen) */
     if (PKCS1_MGF1(seedmask, mdlen, db, dbmask_len, mgf1md) < 0)
         goto err;
+    /* stepo 3h: maskedMGFSeed = mgfSeed XOR mgfSeedMask */
     for (i = 0; i < mdlen; i++)
         seed[i] ^= seedmask[i];
     rv = 1;
@@ -270,6 +287,13 @@ int RSA_padding_check_PKCS1_OAEP_mgf1(unsigned char *to, int tlen,
     return constant_time_select_int(good, mlen, -1);
 }
 
+/*
+ * Mask Generation Function corresponding to section 7.2.2.2 of NIST SP 800-56B.
+ * The variables are named differently to NIST:
+ *      mask (T) and len (maskLen)are the returned mask.
+ *      seed (mgfSeed).
+ * The range checking steps inm the process are performed outside.
+ */
 int PKCS1_MGF1(unsigned char *mask, long len,
                const unsigned char *seed, long seedlen, const EVP_MD *dgst)
 {
@@ -285,11 +309,14 @@ int PKCS1_MGF1(unsigned char *mask, long len,
     mdlen = EVP_MD_size(dgst);
     if (mdlen < 0)
         goto err;
+    /* step 4 */
     for (i = 0; outlen < len; i++) {
+        /* step 4a: D = I2BS(counter, 4) */
         cnt[0] = (unsigned char)((i >> 24) & 255);
         cnt[1] = (unsigned char)((i >> 16) & 255);
         cnt[2] = (unsigned char)((i >> 8)) & 255;
         cnt[3] = (unsigned char)(i & 255);
+        /* step 4b: T =T || hash(mgfSeed || D) */
         if (!EVP_DigestInit_ex(c, dgst, NULL)
             || !EVP_DigestUpdate(c, seed, seedlen)
             || !EVP_DigestUpdate(c, cnt, 4))