# TLS_KRB5_WITH_IDEA_CBC_MD5 cipher suite

> TLS_KRB5_WITH_IDEA_CBC_MD5 (0x00,0x25): KRB5/KRB5 · IDEA-CBC · TLS 1.0, TLS 1.1. ssl.sh rating: Insecure. OpenSSL/GnuTLS names and Nginx/Apache config.

Source: https://ssl.sh/ciphers/tls-krb5-with-idea-cbc-md5/
Last updated: 2026-11-10

TLS_KRB5_WITH_IDEA_CBC_MD5 (0x00,0x25) is a cipher suite for TLS 1.0, TLS 1.1: KRB5 key exchange, KRB5 authentication, IDEA-CBC encryption and MD5 message authentication. ssl.sh rates it Insecure.

| Field | Value |
|---|---|
| IANA | TLS_KRB5_WITH_IDEA_CBC_MD5 |
| Hex code | 0x00,0x25 |
| OpenSSL | Not available in OpenSSL 3.5 |
| GnuTLS | - |
| Key exchange | KRB5 |
| Authentication | KRB5 |
| Encryption | IDEA-CBC |
| MAC | MD5 |
| TLS versions | TLS 1.0, TLS 1.1 |
| Rating | Insecure |
| IANA "Recommended" | D |
| References | RFC 2712, RFC 9847 |

## What each component means

A [cipher suite](https://ssl.sh/glossary/cipher-suite/) names the algorithms a client and server agree on in one handshake. TLS_KRB5_WITH_IDEA_CBC_MD5 combines KRB5 key exchange, KRB5 authentication, IDEA-CBC encryption and MD5 integrity protection. Each part is explained below with its known weaknesses; for the bigger picture see [how cipher suites work](https://ssl.sh/learn/cipher-suites/).

Key exchange and authentication: Kerberos ([RFC 2712](https://www.rfc-editor.org/rfc/rfc2712)). Instead of a certificate exchange, the client presents a Kerberos service ticket and the premaster secret is protected with the ticket's session key, so TLS can reuse existing Kerberos infrastructure. These suites were only ever defined with DES, 3DES, RC4, IDEA and 40-bit export ciphers, so no modern combination exists, and OpenSSL 3.5 implements none of them.

Encryption: IDEA, a 1991 block cipher with a 128-bit key but only a 64-bit block. TLS 1.2 ([RFC 5246](https://www.rfc-editor.org/rfc/rfc5246)) dropped the IDEA and DES suites as deprecated, and the 64-bit block puts it in the same birthday-bound class as 3DES ([Sweet32](https://ssl.sh/glossary/sweet32/)).

Mode: CBC with MAC-then-encrypt. TLS 1.2 and earlier compute the HMAC over the plaintext, then pad and encrypt, so a receiver has to decrypt before it can check integrity. That ordering produced a long run of padding-oracle attacks: [BEAST](https://ssl.sh/glossary/beast/) (CVE-2011-3389) against TLS 1.0's predictable IVs, Lucky Thirteen (CVE-2013-0169, 2013), which times the MAC check, and [POODLE](https://ssl.sh/glossary/poodle/) (CVE-2014-3566, 2014) against SSL 3.0's unchecked padding. [RFC 9325](https://www.rfc-editor.org/rfc/rfc9325) (2022) says CBC suites SHOULD NOT be used unless the encrypt_then_mac extension ([RFC 7366](https://www.rfc-editor.org/rfc/rfc7366)) is also negotiated.

Integrity: HMAC-MD5. MD5 collisions have been practical since 2004; HMAC-MD5 is not directly broken, but MD5 survives only in suites whose ciphers are already obsolete, and [RFC 9155](https://www.rfc-editor.org/rfc/rfc9155) (2021) has since removed MD5 from TLS 1.2 signatures too.

## Why this rating

- Uses IDEA, a 64-bit block cipher that RFC 5246 removed from TLS 1.2.
- Uses HMAC-MD5 for integrity; MD5 is broken and these suites are deprecated.

## Which TLS libraries support it

Support for TLS_KRB5_WITH_IDEA_CBC_MD5 as read from each library's own source code or official documentation on 2026-11-10. “Enabled by default” means the library negotiates it with no configuration; “off by default” means it is implemented but has to be switched on.

| Library | Status | Details |
|---|---|---|
| OpenSSL 3.5 | Not supported | - |
| GnuTLS | Not supported | - |
| BoringSSL | Not supported | - |
| Go crypto/tls | Not supported | - |
| NSS 3.133 | Not supported | - |
| Firefox | Not supported | - |
| Java 21 (SunJSSE) | Not supported | - |
| Windows Schannel (Windows 11 / Server 2022) | Not supported | - |
| rustls 0.23.46 | Not supported | - |

None of the 9 libraries tracked here enables it by default. None of them implements it.

NSS shows the library's own defaults; Firefox switches every NSS suite off at start-up and re-enables only those with a security.ssl3.* or security.tls13.* preference. Java defaults are derived from the jdk.tls.disabledAlgorithms value Oracle documents for JDK 21, which later updates may extend. The OpenSSL and GnuTLS rows only show whether the suite is implemented.

- [BoringSSL — ssl/ssl_cipher.cc (main)](https://boringssl.googlesource.com/boringssl/+/refs/heads/main/ssl/ssl_cipher.cc) (2026-11-10)
- [Go — src/crypto/tls/cipher_suites.go (master)](https://github.com/golang/go/blob/master/src/crypto/tls/cipher_suites.go) (2026-11-10)
- [Go — src/crypto/tls/defaults.go (master)](https://github.com/golang/go/blob/master/src/crypto/tls/defaults.go) (2026-11-10)
- [NSS — lib/ssl/ssl3con.c (as vendored in Firefox)](https://github.com/mozilla-firefox/firefox/blob/main/security/nss/lib/ssl/ssl3con.c) (2026-11-10)
- [NSS — TAG-INFO](https://github.com/mozilla-firefox/firefox/blob/main/security/nss/TAG-INFO) (2026-11-10)
- [Firefox — security/manager/ssl/nsNSSComponent.cpp](https://github.com/mozilla-firefox/firefox/blob/main/security/manager/ssl/nsNSSComponent.cpp) (2026-11-10)
- [Firefox — modules/libpref/init/StaticPrefList.yaml](https://github.com/mozilla-firefox/firefox/blob/main/modules/libpref/init/StaticPrefList.yaml) (2026-11-10)
- [Oracle — JDK 21 Providers Documentation: SunJSSE Cipher Suites](https://docs.oracle.com/en/java/javase/21/security/oracle-providers.html#GUID-7093246A-31A3-4304-AC5F-5FB6400405E2) (2026-11-10)
- [Oracle — JDK 21 JSSE Reference Guide: jdk.tls.disabledAlgorithms](https://docs.oracle.com/en/java/javase/21/security/java-secure-socket-extension-jsse-reference-guide.html) (2026-11-10)
- [Microsoft Learn — TLS Cipher Suites in Windows 11](https://learn.microsoft.com/en-us/windows/win32/secauthn/tls-cipher-suites-in-windows-11) (2026-11-10)
- [Microsoft Learn — TLS Cipher Suites in Windows Server 2022](https://learn.microsoft.com/en-us/windows/win32/secauthn/tls-cipher-suites-in-windows-server-2022) (2026-11-10)
- [docs.rs — rustls 0.23.46 crypto::aws_lc_rs (default provider)](https://docs.rs/rustls/0.23.46/src/rustls/crypto/aws_lc_rs/mod.rs.html) (2026-11-10)

## History and standards status

TLS_KRB5_WITH_IDEA_CBC_MD5 (0x00,0x25) is registered in the IANA TLS Cipher Suites registry with [RFC 2712](https://www.rfc-editor.org/rfc/rfc2712), “Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)” (October 1999, Standards Track) as its specification. The TLS 1.2 specification, [RFC 5246](https://www.rfc-editor.org/rfc/rfc5246) (2008), removed the IDEA and DES suites, calling them deprecated. It can only be negotiated in TLS 1.0, TLS 1.1. [RFC 8996](https://www.rfc-editor.org/rfc/rfc8996) (2021) deprecated TLS 1.0 and TLS 1.1, so no protocol version still in good standing allows it. The registry marks it D (discouraged), a value introduced by [RFC 9847](https://www.rfc-editor.org/rfc/rfc9847) (2025).

## How to test whether a server accepts it

OpenSSL 3.5 cannot offer TLS_KRB5_WITH_IDEA_CBC_MD5, so openssl s_client can't test it directly; use a scanner that implements the suite itself.

nmap's ssl-enum-ciphers script connects repeatedly, trying one suite at a time, and lists every suite the server accepts with a grade from A to F. Look for TLS_KRB5_WITH_IDEA_CBC_MD5 in its output:

```bash
nmap --script ssl-enum-ciphers -p 443 example.com
```

To decode a name or hex code, use the [cipher suite lookup](https://ssl.sh/tools/cipher-suite-lookup/); the [SSL checker](https://ssl.sh/tools/ssl-checker/) shows which protocol versions a live server accepts and the cipher suite it negotiates.

## FAQ

### Is TLS_KRB5_WITH_IDEA_CBC_MD5 secure?

No. ssl.sh rates it insecure. Uses IDEA, a 64-bit block cipher that RFC 5246 removed from TLS 1.2. It can only be negotiated in TLS 1.0, TLS 1.1. [RFC 8996](https://www.rfc-editor.org/rfc/rfc8996) (2021) deprecated TLS 1.0 and TLS 1.1, so no protocol version still in good standing allows it.

### Can TLS_KRB5_WITH_IDEA_CBC_MD5 be used with TLS 1.3?

No. It only works with TLS 1.0, TLS 1.1, all deprecated. TLS 1.3 uses its own suites such as [TLS_AES_128_GCM_SHA256](https://ssl.sh/ciphers/tls-aes-128-gcm-sha256/).

### How do I disable TLS_KRB5_WITH_IDEA_CBC_MD5 in Nginx or Apache?

There is nothing to disable: OpenSSL 3.5, which Nginx and Apache httpd normally use, does not implement TLS_KRB5_WITH_IDEA_CBC_MD5, so they cannot negotiate it.

### Which clients and libraries still support TLS_KRB5_WITH_IDEA_CBC_MD5?

Not supported: OpenSSL 3.5, GnuTLS, BoringSSL, Go crypto/tls, NSS 3.133, Firefox, Java 21 (SunJSSE), Windows Schannel (Windows 11 / Server 2022) and rustls 0.23.46.