Update the demos/README file because it is really old. New demos should provide best practice for API use.

Add demonstration for computing a SHA3-512 digest - digest/EVP_MD_demo

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/14150)

1 files changed, 122 insertions, 0 deletions

diff --git a/demos/digest/BIO_f_md.c b/demos/digest/BIO_f_md.c
new file mode 100755
index 0000000000..ce1dfcc34a
--- /dev/null
+++ b/demos/digest/BIO_f_md.c
@@ -0,0 +1,122 @@
+/*-
+ * Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+/*-
+ * Example of using EVP_MD_fetch and EVP_Digest* methods to calculate
+ * a digest of static buffers
+ * You can find SHA3 test vectors from NIST here:
+ * https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Algorithm-Validation-Program/documents/sha3/sha-3bytetestvectors.zip
+ * For example, contains these lines:
+    Len = 80
+    Msg = 1ca984dcc913344370cf
+    MD = 6915ea0eeffb99b9b246a0e34daf3947852684c3d618260119a22835659e4f23d4eb66a15d0affb8e93771578f5e8f25b7a5f2a55f511fb8b96325ba2cd14816
+ * use xxd convert the hex message string to binary input for BIO_f_md:
+ * echo "1ca984dcc913344370cf" | xxd -r -p | ./BIO_f_md
+ * and then verify the output matches MD above.
+ */
+
+#include <string.h>
+#include <stdio.h>
+#include <openssl/err.h>
+#include <openssl/bio.h>
+#include <openssl/evp.h>
+
+/*-
+ * This demonstration will show how to digest data using
+ * a BIO configured with a message digest
+ * A message digest name may be passed as an argument.
+ * The default digest is SHA3-512
+ */
+
+int main(int argc, char * argv[])
+{
+    int result = 1;
+    OSSL_LIB_CTX *library_context = NULL;
+    BIO *input = NULL;
+    BIO *bio_digest = NULL;
+    EVP_MD *md = NULL;
+    unsigned char buffer[512];
+    size_t readct, writect;
+    size_t digest_size;
+    char *digest_value=NULL;
+    int j;
+
+    input = BIO_new_fd( fileno(stdin), 1 );
+    if (input == NULL) {
+        fprintf(stderr, "BIO_new_fd() for stdin returned NULL\n");
+        goto cleanup;
+    }
+    library_context = OSSL_LIB_CTX_new();
+    if (library_context == NULL) {
+        fprintf(stderr, "OSSL_LIB_CTX_new() returned NULL\n");
+        goto cleanup;
+    }
+
+    /*
+     * Fetch a message digest by name
+     * The algorithm name is case insensitive. 
+     * See providers(7) for details about algorithm fetching
+     */
+    md = EVP_MD_fetch( library_context, "SHA3-512", NULL );
+    if (md == NULL) {
+        fprintf(stderr, "EVP_MD_fetch did not find SHA3-512.\n");
+        goto cleanup;
+    }
+    digest_size = EVP_MD_size(md);
+    digest_value = OPENSSL_malloc(digest_size);
+    if (digest_value == NULL) {
+        fprintf(stderr, "Can't allocate %lu bytes for the digest value.\n", (unsigned long)digest_size);
+        goto cleanup;
+    }
+    /* Make a bio that uses the digest */
+    bio_digest = BIO_new(BIO_f_md());
+    if (bio_digest == NULL) {
+        fprintf(stderr, "BIO_new(BIO_f_md()) returned NULL\n");
+        goto cleanup;
+    }
+    /* set our bio_digest BIO to digest data */
+    if (BIO_set_md(bio_digest,md) != 1) {
+           fprintf(stderr, "BIO_set_md failed.\n");
+           goto cleanup;
+    }
+    /*-
+     * We will use BIO chaining so that as we read, the digest gets updated
+     * See the man page for BIO_push
+     */
+    BIO *reading = BIO_push( bio_digest, input );
+    
+    while( BIO_read(reading, buffer, sizeof(buffer)) > 0 )
+        ;
+
+    /*-
+     * BIO_gets must be used to calculate the final
+     * digest value and then copy it to digest_value.
+     */
+    if (BIO_gets(bio_digest, digest_value, digest_size) != digest_size) {
+        fprintf(stderr, "BIO_gets(bio_digest) failed\n");
+        goto cleanup;
+    }
+    for (j=0; j<digest_size; j++) {
+        fprintf(stdout, "%02x", (unsigned char)digest_value[j]);
+    }
+    fprintf(stdout, "\n");
+    result = 0;
+    
+cleanup:
+    if (result != 0) 
+        ERR_print_errors_fp(stderr);
+
+    OPENSSL_free(digest_value);
+    BIO_free(input);
+    BIO_free(bio_digest);
+    EVP_MD_free(md);
+    OSSL_LIB_CTX_free(library_context);
+
+    return result;
+}