| RSAUTL(1) | OpenSSL | RSAUTL(1) | 
openssl rsautl -sign -in file -inkey key.pem -out sig
Recover the signed data
openssl rsautl -verify -in sig -inkey key.pem
Examine the raw signed data:
openssl rsautl -verify -in sig -inkey key.pem -raw -hexdump 0000 - 00 01 ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0010 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0020 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0030 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0040 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0050 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0060 - ff ff ff ff ff ff ff ff-ff ff ff ff ff ff ff ff ................ 0070 - ff ff ff ff 00 68 65 6c-6c 6f 20 77 6f 72 6c 64 .....hello world
The PKCS#1 block formatting is evident from this. If this was done using encrypt and decrypt the block would have been of type 2 (the second byte) and random padding data visible instead of the 0xff bytes.
It is possible to analyse the signature of certificates using this utility in conjunction with asn1parse. Consider the self signed example in certs/pca-cert.pem . Running asn1parse as follows yields:
 openssl asn1parse -in pca-cert.pem
    0:d=0  hl=4 l= 742 cons: SEQUENCE
    4:d=1  hl=4 l= 591 cons:  SEQUENCE
    8:d=2  hl=2 l=   3 cons:   cont [ 0 ]
   10:d=3  hl=2 l=   1 prim:    INTEGER           :02
   13:d=2  hl=2 l=   1 prim:   INTEGER           :00
   16:d=2  hl=2 l=  13 cons:   SEQUENCE
   18:d=3  hl=2 l=   9 prim:    OBJECT            :md5WithRSAEncryption
   29:d=3  hl=2 l=   0 prim:    NULL
   31:d=2  hl=2 l=  92 cons:   SEQUENCE
   33:d=3  hl=2 l=  11 cons:    SET
   35:d=4  hl=2 l=   9 cons:     SEQUENCE
   37:d=5  hl=2 l=   3 prim:      OBJECT            :countryName
   42:d=5  hl=2 l=   2 prim:      PRINTABLESTRING   :AU
  ....
  599:d=1  hl=2 l=  13 cons:  SEQUENCE
  601:d=2  hl=2 l=   9 prim:   OBJECT            :md5WithRSAEncryption
  612:d=2  hl=2 l=   0 prim:   NULL
  614:d=1  hl=3 l= 129 prim:  BIT STRING
The final BIT STRING contains the actual signature. It can be extracted with:
openssl asn1parse -in pca-cert.pem -out sig -noout -strparse 614
The certificate public key can be extracted with:
openssl x509 -in test/testx509.pem -pubkey -noout >pubkey.pem
The signature can be analysed with:
 openssl rsautl -in sig -verify -asn1parse -inkey pubkey.pem -pubin
    0:d=0  hl=2 l=  32 cons: SEQUENCE
    2:d=1  hl=2 l=  12 cons:  SEQUENCE
    4:d=2  hl=2 l=   8 prim:   OBJECT            :md5
   14:d=2  hl=2 l=   0 prim:   NULL
   16:d=1  hl=2 l=  16 prim:  OCTET STRING
      0000 - f3 46 9e aa 1a 4a 73 c9-37 ea 93 00 48 25 08 b5   .F...Js.7...H%..
This is the parsed version of an ASN1 DigestInfo structure. It can be seen that the digest used was md5. The actual part of the certificate that was signed can be extracted with:
openssl asn1parse -in pca-cert.pem -out tbs -noout -strparse 4
and its digest computed with:
openssl md5 -c tbs MD5(tbs)= f3:46:9e:aa:1a:4a:73:c9:37:ea:93:00:48:25:08:b5
which it can be seen agrees with the recovered value above.
Licensed under the OpenSSL license (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>.
| 2018-09-23 | 1.1.1i |