Create Authorization Basic Header

The HTTP Authorization request header is sometimes required to authenticate a user agent with a server.
This post explains how to create the header on linux at command line.

Syntax

The HTTP Authorization request header has the following syntax:

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Authorization: <type> <credentials>

The type is typically “Basic”, in which case the credentials are of the form user:password encoded as base64.

Curl will generate this header for us if we use the -u option:

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$ curl -v -u user:password majgis.github.io     
...
> Authorization: Basic dXNlcjpwYXNzd29yZA==
...

Now for the real question, how do we generate this header for use with curl’s -H option?

Experiments

Here is the first attempt to base64 encode user:password that is WRONG:

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# This is WRONG!
$ echo user:password | base64 
dXNlcjpwYXNzd29yZAo=

Notice that dXNlcjpwYXNzd29yZAo= does not equal dXNlcjpwYXNzd29yZA==.

Let’s decode to find the difference:

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$ echo dXNlcjpwYXNzd29yZAo= | base64 -d
user:password
$ echo dXNlcjpwYXNzd29yZA== | base64 -d
user:password%

That % is different. What is that?

The % symbol is how zsh handles the end of partial lines.
In other words, zsh assumes we want our prompt on a newline even if the last command didn’t end with
a newline. That % symbol is how we know the difference between a newline the last command output
and a newline zsh graciously inserted for us.

Let’s execute this command under bash and see the difference:

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bash
$ echo dXNlcjpwYXNzd29yZAo= | base64 -d
user:password
$ echo dXNlcjpwYXNzd29yZA== | base64 -d
user:password$

The base64 encoded user:password that curl generated is not terminated with a newline, unlike the one we generated.

Let’s see if we can get echo to not add that newline.

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$ man echo | cat | grep 'newline' 
       -n     do not output the trailing newline

The -n option for echo is what we want.

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$ echo -n user:password | base64
dXNlcjpwYXNzd29yZA==

We see that the output is exactly the same as what curl generated.

Putting it all together:

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$ curl -v -H "Authorization: Basic `echo -n user:password | base64`" majgis.github.io
...
> Authorization: Basic dXNlcjpwYXNzd29yZA==
...

Take Away

In zsh, a % symbol is placed at the end of a line where zsh inserted a newline for us.

It is important to include echo’s -n option when piping content to an encoder to avoid a
newline character being included in the encoded output.

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$ echo -n user:password | base64

Experimental Setup

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$ echo && curl --version && echo && lsb_release -d -c && echo && echo $0 && zsh --version
curl 7.52.1 (x86_64-pc-linux-gnu) libcurl/7.52.1 OpenSSL/1.0.2g zlib/1.2.11 libidn2/0.16 libpsl/0.17.0 (+libidn2/0.16) librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp 
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP UnixSockets HTTPS-proxy PSL 
Description:    Ubuntu 17.04
Codename:       zesty
/usr/bin/zsh
zsh 5.2 (x86_64-ubuntu-linux-gnu)

References