#Stax, 15 bytes

┐°!'àÉgYg8∟╗`╖ë 

Run and debug it online

This 15 byte submission is packed into a variant of the CP437 character set. The corresponding ascii representation takes 18 bytes:

EVlVd+26/s@:fs{[Io 

Pretty sure it can be further trimmed down though.

E Put the two inputs on main stack Vl "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" Vd "0123456789" + Concatenate 26/ Partition to blocks of size 26 (array `a`) s@ Index array `a` with the input index array :fs Flatten to get a string `s` {[Io Order the input string Using the char array `s` as the key Implicit output 

VlVd+ can also be VLA|(, which left rotates the 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ by ten elemenets. The whole code can also be VlVd+26/,e@:f,{[Io, which reads the input stack twice instead of reading them all at the beginning to the main stack, and uses a different (more traditional) input format, as shown in this.

Stax, 15 bytes

┐°!'àÉgYg8∟╗`╖ë 

Run and debug it online

This 15 byte submission is packed into a variant of the CP437 character set. The corresponding ascii representation takes 18 bytes:

EVlVd+26/s@:fs{[Io 

Pretty sure it can be further trimmed down though.

E Put the two inputs on main stack Vl "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" Vd "0123456789" + Concatenate 26/ Partition to blocks of size 26 (array `a`) s@ Index array `a` with the input index array :fs Flatten to get a string `s` {[Io Order the input string Using the char array `s` as the key Implicit output 

VlVd+ can also be VLA|(, which left rotates the 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ by ten elemenets. The whole code can also be VlVd+26/,e@:f,{[Io, which reads the input stack twice instead of reading them all at the beginning to the main stack, and uses a different (more traditional) input format, as shown in this.

#Stax, 15 bytes

┐°!'àÉgYg8∟╗`╖ë 

Run and debug it online

This 15 byte submission is packed into a variant of the CP437 character set. The corresponding ascii representation takes 18 bytes:

EVlVd+26/s@:fs{[Io 

Pretty sure it can be further trimmed down though.

E Put the two inputs on main stack Vl "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" Vd "0123456789" + Concatenate 26/ Partition to blocks of size 26 (array `a`) s@ Index array `a` with the input index array :fs Flatten to get a string `s` {[Io Order the input string Using the char array `s` as the key Implicit output 

VlVd+ can also be VLA|(, which left rotates the 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ by ten elemenets. The whole code can also be VlVd+26/,e@:f,{[Io, which reads the input stack twice instead of reading them all at the beginning to the main stack, and uses a different (more traditional) input format, as shown in this.

#Stax, 15 bytes

┐°!'àÉgYg8∟╗`╖ë 

Run and debug it online

This 15 byte submission is packed into a variant of the CP437 character set. The corresponding ascii representation takes 18 bytes:

EVlVd+26/s@:fs{[Io 

Pretty sure it can be further trimmed down though.

E Put the two inputs on main stack Vl "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" Vd "0123456789" + Concatenate 26/ Partition to blocks of size 26 (array `a`) s@ Index array `a` with the input index array :fs Flatten to get a string `s` {[Io Order the input string Using the char array `s` as the key Implicit output 

VlVd+ can also be VLA|(, which left rotates the 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ by ten elemenets. The whole code can also be VlVd+26/,e@:f,{[Io, which reads the input stack twice instead of reading them all at the beginning to the main stack, and uses a different (more traditional) input format, as shown in this.