OPENSSL-RSAUTL(1ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 OPENSSL-RSAUTL — the RSA command (OpenSSL manual, page OPENSSL-RSAUTL(1ossl))

okay so bestie, before you get excited: it's deprecated. like, this page is a nostalgia tour. but the lore is genuinely useful, and also openssl-pkeyutl(1) is the glow-up you should actually be using. iykyk 🫶

✨ SYNOPSIS ✨

openssl rsautl [-help] [-in file] [-passin arg] [-rev] [-out file]
[-inkey filename|uri] [-keyform DER|PEM|P12|ENGINE] [-pubin] [-certin]
[-sign] [-verify] [-encrypt] [-decrypt] [-pkcs] [-x931] [-oaep] [-raw]
[-hexdump] [-asn1parse] [-engine id] [-rand files] [-writerand file]
[-provider name] [-provider-path path] [-provparam [name:]key=value]
[-propquery propq]

🌸 DESCRIPTION 🌸

hi so first thing: this command has been deprecated. 😭 you should be using the openssl-pkeyutl(1) command instead. that's the main character now, rsautl is the older sibling living in the past.

that said, this command can be used to sign, verify, encrypt and decrypt data using the RSA algorithm. slay, conceptually. just, y'know, deprecated.

🎀 OPTIONS 🎀

  • -help — prints a usage message. the "what do i even do here" button. 💖
  • -in filename — the input filename to read data from. if you don't specify it, it reads from standard input. no filename? stdin it is, bestie.
  • -passin arg — the passphrase used in the output file. see openssl-passphrase-options(1). (yes the page literally says "See see" 💀 i'm being normal about it)
  • -rev — reverses the order of the input. flip it and reverse it. 🦋
  • -out filename — the output filename to write to; standard output by default.
  • -inkey filename|uri — the input key. by default it should be an RSA private key.
  • -keyform DER|PEM|P12|ENGINE — the key format. unspecified by default. see openssl-format-options(1) for the details, they did that whole explainer already.
  • -pubin — by default a private key is read from the key input. with this option a public key is read instead. and if the input contains no public key but a private key, its public part is used. bestie is resourceful like that.
  • -certin — the input is a certificate containing an RSA public key. 🎫
  • -sign — signs the input data and outputs the signed result. this requires an RSA private key. no private key, no signing, periodt.
  • -verify — verifies the input data and outputs the recovered data.
  • -encrypt — encrypts the input data using an RSA public key.
  • -decrypt — decrypts the input data using an RSA private key.
  • -pkcs — padding: PKCS#1 v1.5, which is the default. (for signatures, only -pkcs and -raw can be used)
  • -oaep — padding: PKCS#1 OAEP. (for signatures, only -pkcs and -raw can be used)
  • -x931 — padding: ANSI X9.31. (for signatures, only -pkcs and -raw can be used)
  • -raw — padding: no padding. the feral option. (for signatures, only -pkcs and -raw can be used)
⚠️ okay important note, no skipping: because of protection against Bleichenbacher attacks, decryption using PKCS#1 v1.5 mode will not return errors in case the padding check failed. so if you actually want to know whether the padding is correct, use -raw and inspect the returned value manually. like, with your own eyeballs. 👀

🦋 NOTES 🦋

since this command uses the RSA algorithm directly, it can only be used to sign or verify small pieces of data. it is not the tool for your 4GB video file, bestie, please. 😭

🌈 EXAMPLES 🌈

examples equivalent to these can be found in the documentation for the non-deprecated openssl-pkeyutl(1) command. so like, these are the vintage editions. 🎀

sign some data using a private key:

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. iconique difference, honestly.

it is possible to analyse the signature of certificates using this command in conjunction with openssl-asn1parse(1). consider the self signed example in certs/pca-cert.pem. running openssl-asn1parse(1) 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. the math is mathing. 🔥

👑 SEE ALSO 👑

openssl(1), openssl-pkeyutl(1), openssl-dgst(1), openssl-rsa(1), openssl-genrsa(1)

⭐ HISTORY ⭐

  • this command was deprecated in OpenSSL 3.0.
  • the -engine option was deprecated in OpenSSL 3.0.

Copyright 2000-2023 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>.

and that's the whole page bestie 🫶 this edition is OpenSSL 3.5.0, dated 2026-09-25, filed under OPENSSL-RSAUTL(1ossl) — now go use openssl-pkeyutl(1) like the icon you are. 💅✨🎀

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