I'm trying to figure out why I'm getting the error 140079157163688:error:14082174:SSL routines:SSL3_CHECK_CERT_AND_ALGORITHM:dh key too small:s3_clnt.c:3435: when connecting to, in my case, a POP3 mail server.

Here's an OpenSSL s_client dump when trying to connect. The error is right before the certificate chain dump.

$ openssl s_client -connect fqdn.example.com:pop3 -starttls pop3
CONNECTED(00000003)
depth=3 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root
verify return:1
depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority
verify return:1
depth=1 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Domain Validation Secure Server CA
verify return:1
depth=0 OU = Domain Control Validated, OU = PositiveSSL Multi-Domain, CN = another-fqdn.example.com
verify return:1
140079157163688:error:14082174:SSL routines:SSL3_CHECK_CERT_AND_ALGORITHM:dh key too small:s3_clnt.c:3435:
---
Certificate chain
 0 s:/OU=Domain Control Validated/OU=PositiveSSL Multi-Domain/CN=another-fqdn.example.com
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
 1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
 2 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
   i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
---
Server certificate
-----BEGIN CERTIFICATE-----
...elided...
-----END CERTIFICATE-----
subject=/OU=Domain Control Validated/OU=PositiveSSL Multi-Domain/CN=another-fqdn.example.com
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
---
No client certificate CA names sent
---
SSL handshake has read 5005 bytes and written 7 bytes
---
New, (NONE), Cipher is (NONE)
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : 0000
    Session-ID: 
    Session-ID-ctx: 
    Master-Key: 
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1483127963
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---

Here's an unrelated bug report which points toward the problem being a too short ephemeral key (Server Temp Key: DH, 512 bits). But I'm not seeing anything like that in my output, even when adding the -debug parameter to the OpenSSL invocation.

I want to find out exactly which key is "too small" so that I can either fix it locally, or forward it to whoever is in a position to fix it and give them enough information that they should be able to fix it without too much trouble. So how do I find out which key exactly OpenSSL is complaining about?

I'm willing to use other tools as well for investigating this; answers need not be limited to using OpenSSL.

up vote 2 down vote accepted

I want to find out exactly which key is "too small" so that I can either fix it locally, or forward it to whoever is in a position to fix it and give them enough information that they should be able to fix it without too much trouble. So how do I find out which key exactly OpenSSL is complaining about?

Its not really a problem of "one key is too small, which one is it" per se. When you perform key agreement in TLS, you do so using Diffie-Hellman domain parameters. When the client advertises cipher suites with Diffie-Hellman in its Client Hello, the Server Hello responds with a Diffie-Hellman group and its ephemeral/temporary public key in that group. As a client, you are expected to execute the protocol using the server's ephemeral/temporary public key, and communicate your ephemeral/temporary public key to the server in the next message.

More formally, you are talking about a multiplicative group Zp. The server chooses a secret value X, computes its ephemeral/temporary public key as x = gX mod p, and sends you the domain parameters {p, g} and its public key x. You would calculate the ephemeral/temporary public key as y = gY mod p, and the client/server agreed upon secret drops out of gXY mod p after a few messages.

The complaint is that p is too small, it does not provide anything close to reasonable security levels, and its subject to being brute forced. So whatever the server calculates (x = gX mod p) and whatever you calculate (y = gY mod p) does not achieve reasonable security. For the details, see the paper on the Logjam attack.

There are other attacks in these Diffie-Hellman agreements as used in TLS, but this is one of the bigger ones. For example, the TLS working group is discussing re-use of server side precomputed DH keys as we speak at [TLS] Requiring that (EC)DHE public values be fresh.


If you want to view the actual parameters that are causing the trouble, then you will probably need to run Wireshark or use tcpdump. s_client displays Server Temp Key info only on successful handshake (and only in version 1.0.2 up, but as of this month that's the lowest version supported upstream). It will dump the protocol messages (sent and) received in hex if you add -msg, but you have to decode them by hand which isn't easy; if you want to try see this SO question.


You have three fixes available that I am aware.

First, change email hosting providers to one which is more astute with respect to security.

Second, write to your current email hosting provider and ask them to fix their server.

Third, you can use RSA key transport rather than Diffie-hellman key exchange. If you choose this option, then you lose forward secrecy. The TLS working group is dropping RSA key transport from TLS 1.3, so you won't be able to use this option with future versions of TLS. I expect TLS 1.2 to hang around for some time, so it probably won't be a problem in practice for some time. OpenSSL normally allows this, but some people disable it to force forward secrecy; if you have a cipherlist that specifies !kRSA remove it (temporarily).


If interested, you can see the code points IANA reserves for TLS cipher suites using Diffie-Hellman at Transport Layer Security (TLS) Parameters.

You can get OpenSSL to list the suites it implements and enables by default, both of which can vary with your version and sometimes build/package, with:

$ openssl ciphers -v | egrep 'Kx=(ECDH|DH)'
ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(256) Mac=AEAD
ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA384
ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA384
ECDHE-RSA-AES256-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(256)  Mac=SHA1
ECDHE-ECDSA-AES256-SHA  SSLv3 Kx=ECDH     Au=ECDSA Enc=AES(256)  Mac=SHA1
DH-DSS-AES256-GCM-SHA384 TLSv1.2 Kx=DH/DSS   Au=DH   Enc=AESGCM(256) Mac=AEAD
DHE-DSS-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=DSS  Enc=AESGCM(256) Mac=AEAD
DH-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH/RSA   Au=DH   Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(256) Mac=AEAD
DHE-RSA-AES256-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA256
DHE-DSS-AES256-SHA256   TLSv1.2 Kx=DH       Au=DSS  Enc=AES(256)  Mac=SHA256
DH-RSA-AES256-SHA256    TLSv1.2 Kx=DH/RSA   Au=DH   Enc=AES(256)  Mac=SHA256
DH-DSS-AES256-SHA256    TLSv1.2 Kx=DH/DSS   Au=DH   Enc=AES(256)  Mac=SHA256
DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA1
DHE-DSS-AES256-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(256)  Mac=SHA1
DH-RSA-AES256-SHA       SSLv3 Kx=DH/RSA   Au=DH   Enc=AES(256)  Mac=SHA1
DH-DSS-AES256-SHA       SSLv3 Kx=DH/DSS   Au=DH   Enc=AES(256)  Mac=SHA1
DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH       Au=RSA  Enc=Camellia(256) Mac=SHA1
DHE-DSS-CAMELLIA256-SHA SSLv3 Kx=DH       Au=DSS  Enc=Camellia(256) Mac=SHA1
DH-RSA-CAMELLIA256-SHA  SSLv3 Kx=DH/RSA   Au=DH   Enc=Camellia(256) Mac=SHA1
DH-DSS-CAMELLIA256-SHA  SSLv3 Kx=DH/DSS   Au=DH   Enc=Camellia(256) Mac=SHA1
ECDH-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(256) Mac=AEAD
ECDH-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AESGCM(256) Mac=AEAD
ECDH-RSA-AES256-SHA384  TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA384
ECDH-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256)  Mac=SHA384
ECDH-RSA-AES256-SHA     SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(256)  Mac=SHA1
ECDH-ECDSA-AES256-SHA   SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256)  Mac=SHA1
ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AESGCM(128) Mac=AEAD
ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA256
ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA256
ECDHE-RSA-AES128-SHA    SSLv3 Kx=ECDH     Au=RSA  Enc=AES(128)  Mac=SHA1
ECDHE-ECDSA-AES128-SHA  SSLv3 Kx=ECDH     Au=ECDSA Enc=AES(128)  Mac=SHA1
DH-DSS-AES128-GCM-SHA256 TLSv1.2 Kx=DH/DSS   Au=DH   Enc=AESGCM(128) Mac=AEAD
DHE-DSS-AES128-GCM-SHA256 TLSv1.2 Kx=DH       Au=DSS  Enc=AESGCM(128) Mac=AEAD
DH-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH/RSA   Au=DH   Enc=AESGCM(128) Mac=AEAD
DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH       Au=RSA  Enc=AESGCM(128) Mac=AEAD
DHE-RSA-AES128-SHA256   TLSv1.2 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA256
DHE-DSS-AES128-SHA256   TLSv1.2 Kx=DH       Au=DSS  Enc=AES(128)  Mac=SHA256
DH-RSA-AES128-SHA256    TLSv1.2 Kx=DH/RSA   Au=DH   Enc=AES(128)  Mac=SHA256
DH-DSS-AES128-SHA256    TLSv1.2 Kx=DH/DSS   Au=DH   Enc=AES(128)  Mac=SHA256
DHE-RSA-AES128-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA1
DHE-DSS-AES128-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(128)  Mac=SHA1
DH-RSA-AES128-SHA       SSLv3 Kx=DH/RSA   Au=DH   Enc=AES(128)  Mac=SHA1
DH-DSS-AES128-SHA       SSLv3 Kx=DH/DSS   Au=DH   Enc=AES(128)  Mac=SHA1
DHE-RSA-SEED-SHA        SSLv3 Kx=DH       Au=RSA  Enc=SEED(128) Mac=SHA1
DHE-DSS-SEED-SHA        SSLv3 Kx=DH       Au=DSS  Enc=SEED(128) Mac=SHA1
DH-RSA-SEED-SHA         SSLv3 Kx=DH/RSA   Au=DH   Enc=SEED(128) Mac=SHA1
DH-DSS-SEED-SHA         SSLv3 Kx=DH/DSS   Au=DH   Enc=SEED(128) Mac=SHA1
DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH       Au=RSA  Enc=Camellia(128) Mac=SHA1
DHE-DSS-CAMELLIA128-SHA SSLv3 Kx=DH       Au=DSS  Enc=Camellia(128) Mac=SHA1
DH-RSA-CAMELLIA128-SHA  SSLv3 Kx=DH/RSA   Au=DH   Enc=Camellia(128) Mac=SHA1
DH-DSS-CAMELLIA128-SHA  SSLv3 Kx=DH/DSS   Au=DH   Enc=Camellia(128) Mac=SHA1
ECDH-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(128) Mac=AEAD
ECDH-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AESGCM(128) Mac=AEAD
ECDH-RSA-AES128-SHA256  TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(128)  Mac=SHA256
ECDH-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(128)  Mac=SHA256
ECDH-RSA-AES128-SHA     SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(128)  Mac=SHA1
ECDH-ECDSA-AES128-SHA   SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(128)  Mac=SHA1
ECDHE-RSA-RC4-SHA       SSLv3 Kx=ECDH     Au=RSA  Enc=RC4(128)  Mac=SHA1
ECDHE-ECDSA-RC4-SHA     SSLv3 Kx=ECDH     Au=ECDSA Enc=RC4(128)  Mac=SHA1
ECDH-RSA-RC4-SHA        SSLv3 Kx=ECDH/RSA Au=ECDH Enc=RC4(128)  Mac=SHA1
ECDH-ECDSA-RC4-SHA      SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=RC4(128)  Mac=SHA1
ECDHE-RSA-DES-CBC3-SHA  SSLv3 Kx=ECDH     Au=RSA  Enc=3DES(168) Mac=SHA1
ECDHE-ECDSA-DES-CBC3-SHA SSLv3 Kx=ECDH     Au=ECDSA Enc=3DES(168) Mac=SHA1
EDH-RSA-DES-CBC3-SHA    SSLv3 Kx=DH       Au=RSA  Enc=3DES(168) Mac=SHA1
EDH-DSS-DES-CBC3-SHA    SSLv3 Kx=DH       Au=DSS  Enc=3DES(168) Mac=SHA1
DH-RSA-DES-CBC3-SHA     SSLv3 Kx=DH/RSA   Au=DH   Enc=3DES(168) Mac=SHA1
DH-DSS-DES-CBC3-SHA     SSLv3 Kx=DH/DSS   Au=DH   Enc=3DES(168) Mac=SHA1
ECDH-RSA-DES-CBC3-SHA   SSLv3 Kx=ECDH/RSA Au=ECDH Enc=3DES(168) Mac=SHA1
ECDH-ECDSA-DES-CBC3-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=3DES(168) Mac=SHA1

If you use -V uppercase instead of -v it includes the codepoints (1.0.0 up), but the resulting lines are too long for a traditional 80-column window; either use a wider window, or less -S or similar, or redirect to a file and then view or edit the file.

Note that list includes static-DH and static-ECDH suites (except in 1.1.0?) which are practically never used; for a list limited to usable ephemeral suites do ciphers -v 'EDH:EECDH:!EXPORT:!LOW'.

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