func ReadMessage()

in openpgp/read.go [92:226]
111 lines of code
34 McCabe index (conditional complexity)

func ReadMessage(r io.Reader, keyring KeyRing, prompt PromptFunction, config *packet.Config) (md *MessageDetails, err error) {
	var p packet.Packet

	var symKeys []*packet.SymmetricKeyEncrypted
	var pubKeys []keyEnvelopePair
	var se *packet.SymmetricallyEncrypted

	packets := packet.NewReader(r)
	md = new(MessageDetails)
	md.IsEncrypted = true

	// The message, if encrypted, starts with a number of packets
	// containing an encrypted decryption key. The decryption key is either
	// encrypted to a public key, or with a passphrase. This loop
	// collects these packets.
ParsePackets:
	for {
		p, err = packets.Next()
		if err != nil {
			return nil, err
		}
		switch p := p.(type) {
		case *packet.SymmetricKeyEncrypted:
			// This packet contains the decryption key encrypted with a passphrase.
			md.IsSymmetricallyEncrypted = true
			symKeys = append(symKeys, p)
		case *packet.EncryptedKey:
			// This packet contains the decryption key encrypted to a public key.
			md.EncryptedToKeyIds = append(md.EncryptedToKeyIds, p.KeyId)
			switch p.Algo {
			case packet.PubKeyAlgoRSA, packet.PubKeyAlgoRSAEncryptOnly, packet.PubKeyAlgoElGamal:
				break
			default:
				continue
			}
			var keys []Key
			if p.KeyId == 0 {
				keys = keyring.DecryptionKeys()
			} else {
				keys = keyring.KeysById(p.KeyId)
			}
			for _, k := range keys {
				pubKeys = append(pubKeys, keyEnvelopePair{k, p})
			}
		case *packet.SymmetricallyEncrypted:
			se = p
			break ParsePackets
		case *packet.Compressed, *packet.LiteralData, *packet.OnePassSignature:
			// This message isn't encrypted.
			if len(symKeys) != 0 || len(pubKeys) != 0 {
				return nil, errors.StructuralError("key material not followed by encrypted message")
			}
			packets.Unread(p)
			return readSignedMessage(packets, nil, keyring)
		}
	}

	var candidates []Key
	var decrypted io.ReadCloser

	// Now that we have the list of encrypted keys we need to decrypt at
	// least one of them or, if we cannot, we need to call the prompt
	// function so that it can decrypt a key or give us a passphrase.
FindKey:
	for {
		// See if any of the keys already have a private key available
		candidates = candidates[:0]
		candidateFingerprints := make(map[string]bool)

		for _, pk := range pubKeys {
			if pk.key.PrivateKey == nil {
				continue
			}
			if !pk.key.PrivateKey.Encrypted {
				if len(pk.encryptedKey.Key) == 0 {
					pk.encryptedKey.Decrypt(pk.key.PrivateKey, config)
				}
				if len(pk.encryptedKey.Key) == 0 {
					continue
				}
				decrypted, err = se.Decrypt(pk.encryptedKey.CipherFunc, pk.encryptedKey.Key)
				if err != nil && err != errors.ErrKeyIncorrect {
					return nil, err
				}
				if decrypted != nil {
					md.DecryptedWith = pk.key
					break FindKey
				}
			} else {
				fpr := string(pk.key.PublicKey.Fingerprint[:])
				if v := candidateFingerprints[fpr]; v {
					continue
				}
				candidates = append(candidates, pk.key)
				candidateFingerprints[fpr] = true
			}
		}

		if len(candidates) == 0 && len(symKeys) == 0 {
			return nil, errors.ErrKeyIncorrect
		}

		if prompt == nil {
			return nil, errors.ErrKeyIncorrect
		}

		passphrase, err := prompt(candidates, len(symKeys) != 0)
		if err != nil {
			return nil, err
		}

		// Try the symmetric passphrase first
		if len(symKeys) != 0 && passphrase != nil {
			for _, s := range symKeys {
				key, cipherFunc, err := s.Decrypt(passphrase)
				if err == nil {
					decrypted, err = se.Decrypt(cipherFunc, key)
					if err != nil && err != errors.ErrKeyIncorrect {
						return nil, err
					}
					if decrypted != nil {
						break FindKey
					}
				}

			}
		}
	}

	md.decrypted = decrypted
	if err := packets.Push(decrypted); err != nil {
		return nil, err
	}
	return readSignedMessage(packets, md, keyring)
}