acme: implement Client.ListCertAlternates

Let's Encrypt is defaulting to a longer cross-signed chain on May 4th,
2021 but will offer the ability to download the shorter chain via an
alternate URL via a link header [1]. The shorter chain can be selected
to workaround a validation bug in legacy versions of OpenSSL, GnuTLS,
and LibreSSL. The alternate relation is described in section 7.4.2 of
RFC 8555.

ListCertAlternates should be passed the original certificate chain URL
and will return a list of alternate chain URLs that can be passed to
FetchCert to download.

Fixes golang/go#42437


Change-Id: Iaa32e49cb1322ac79ac1a5b4b7980d5401f4b86e
Trust: Filippo Valsorda <>
Run-TryBot: Filippo Valsorda <>
Reviewed-by: Roland Shoemaker <>
TryBot-Result: Go Bot <>
2 files changed
tree: d26f50954113cc7d02d918c073af488fd51a4de5
  1. acme/
  2. argon2/
  3. bcrypt/
  4. blake2b/
  5. blake2s/
  6. blowfish/
  7. bn256/
  8. cast5/
  9. chacha20/
  10. chacha20poly1305/
  11. cryptobyte/
  12. curve25519/
  13. ed25519/
  14. hkdf/
  15. internal/
  16. md4/
  17. nacl/
  18. ocsp/
  19. openpgp/
  20. otr/
  21. pbkdf2/
  22. pkcs12/
  23. poly1305/
  24. ripemd160/
  25. salsa20/
  26. scrypt/
  27. sha3/
  28. ssh/
  29. tea/
  30. twofish/
  31. xtea/
  32. xts/
  33. .gitattributes
  34. .gitignore
  36. codereview.cfg
  39. go.mod
  40. go.sum

Go Cryptography

Go Reference

This repository holds supplementary Go cryptography libraries.


The easiest way to install is to run go get -u You can also manually git clone the repository to $GOPATH/src/

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