mirror of
https://gitlab.kit.edu/kit/scc/sys/mail/exim-encrypt-dlfunc.git
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112 lines
4.4 KiB
Markdown
112 lines
4.4 KiB
Markdown
# exim-encrypt-dlfunc
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This library injects functions for string encryption and decryption into [exim4](https://www.exim.org/). It is basically
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a little glue code to parts of the [libsodium library](https://github.com/jedisct1/libsodium)
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to exim at runtime.
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## Installation
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These instructions are currently only tested on Debian Linux.
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1. Install development tools and libsodium development files:
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```shell
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apt-get install build-essential exim4-dev libsodium-dev
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```
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2. Clone this repository:
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```shell
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git clone https://git.scc.kit.edu/mail/exim-encrypt-dlfunc.git
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cd exim-encrypt-dlfunc/src
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```
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3. Build `genkey` if needed (see below for an explanation):
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```shell
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make genkey
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```
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4. Build the library:
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```shell
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make
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```
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5. Copy to final destination (feel free to pick another place than `/usr/local/lib/`):
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```shell
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sudo install --group=Debian-exim --owner=Debian-exim libexim-encrypt-dlfunc.so /usr/local/lib/
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```
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6. Ensure you have the correct exim flavor:
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Not every flavor of exim is able to load libraries at runtime. Please refer to the
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[documentation](https://www.exim.org/exim-html-current/doc/html/spec_html/ch-string_expansions.html)
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of the `${dlfunc{…}}` function for details. exim from the debian package `exim4-daemon-heavy` meets all the
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requirements.
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## Usage
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There are currently two pairs of functions:
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Symmetric encryption that derives its key from an ASCII string:
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* `sodium_crypto_secretbox_encrypt_password(password, cleartext) → ciphertext`
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* `sodium_crypto_secretbox_decrypt_password(password, ciphertext) → cleartext`
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The generated key is only as strong as the provided password.
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Public key encryption that uses a key pair that needs to be created beforehand:
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* `sodium_crypto_box_seal(public key, cleartext) → ciphertext`
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* `sodium_crypto_box_seal_open(private key, public key, ciphertext) → cleartext`
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The second pair needs a proper key pair in the correct format. This is what the
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`genkey` utility is for. Simply run it once to generate a pair. Be aware that every invocation will overwrite the
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previous key pair without confirmation! Please save both parts in a safe place before proceeding.
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```shell
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$ ./genkey
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=== Creating cryptobox key pair ===
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Wrote »cryptobox_recipient_pk_exim.conf«
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Wrote »cryptobox_recipient_pk.raw«
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Wrote »cryptobox_recipient_sk_exim.conf«
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Wrote »cryptobox_recipient_sk.raw
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```
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The `*_exim.conf` files contain the keys in a format that can simply be pasted into
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`exim.conf` (the first line contains the key as C code and can usually be discarded):
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```shell
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$ cat cryptobox_recipient_pk_exim.conf
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# const unsigned char cryptobox_recipient_pk[32] = { 0xda, 0x46, 0xc8, 0x75, 0x2b, 0x31, 0xd9, 0x0c, 0x83, 0x54, 0x2d, 0x18, 0xda, 0xdc, 0xe5, 0x2d, 0x0e, 0x10, 0xe8, 0x0c, 0x39, 0xde, 0xaf, 0x30, 0x7e, 0xab, 0xca, 0x4d, 0xed, 0x26, 0x4d, 0x6e }; const unsigned int cryptobox_recipient_pk_length = 32;
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cryptobox_recipient_pk = "2kbIdSsx2QyDVC0Y2tzlLQ4Q6Aw53q8wfqvKTe0mTW4="
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```
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The `*.raw` files contain the same key without any formatting; these files are not needed for usage with exim but are
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generated as convenience when writing your own tools.
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### Example: remove `X-Originating-IP:` header
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This example's use case was the initial reason to develop this library: remove the X-Originating-IP header to preserve
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our user's privacy but also keep the information in the final e-mail to enable response to complaints and abuse (the
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original header is usually provided in these cases). Add this snippet to your DATA ACL section in exim:
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```exim
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warn log_message = Removing X-Originating-IP header
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condition = ${if def:h_X-originating-IP: {1}{0}}
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add_header = X-KIT-Orig-IP-PKK: ${dlfunc{/usr/local/lib/libexim-encrypt-dlfunc.so} \
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{sodium_crypto_box_seal} \
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{ktp1OEEItrgvSfpVTtu+ybyNjzuuN8OzCdfrGAJt4j8=} \
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{$h_X-originating-IP:}}
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add_header = X-KIT-Orig-IP-Pass: ${dlfunc{/usr/local/lib/libexim-encrypt-dlfunc.so} \
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{sodium_crypto_secretbox_encrypt_password} \
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{This is a very non-secret key} \
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{$h_X-originating-IP:}}
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remove_header = X-Originating-IP
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```
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Pick one of the `add_header` lines depending on which kind of encryption you want.
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