Python 2, 61 bytes

E=lambda x:x[::2]+x[1::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

E encrypts and D decrypts. I'm not counting the E= and D= for the score.

The decryption takes every n'th character wrapping around, where n is half the string length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so taking every nth character inverts taking every 2nd one.

If using a single function were allowed, I could do 44 bytes

def F(x,b):n=1+len(x)**b>>b;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True. The expression 1+len(x)**b>>b is equal to [2,len(x)/2+1][b].

Python 2, 57 bytes

E=lambda x:(x+x)[::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

Try it online!

Thanks to l4m2 for 4 bytes.

E encrypts and D decrypts. I'm not counting the E= and D= for the score.

The decryption takes every n'th character wrapping around, where n is half the string length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so taking every nth character inverts taking every 2nd one.

If using a single function were allowed, I could do 44 bytes

def F(x,b):n=1+len(x)**b>>b;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True. The expression 1+len(x)**b>>b is equal to [2,len(x)/2+1][b].

Python 2, 61 bytes

E=lambda x:x[::2]+x[1::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

The first one encrypts, the second one decrypts. I'm not counting the E= and D= for the score. The idea of the decryption is to take every n'th character wrapping around, where n half the length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so multiplying the index by n reverses multiplying by 2.

If I were allowed to combine the two functions into one, I could do 44 bytes

def F(x,b):n=1+len(x)**b>>b;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True. The expression 1+len(x)**b>>b is equal to [2,len(x)/2+1][b].

Python 2, 61 bytes

E=lambda x:x[::2]+x[1::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

E encrypts and D decrypts. I'm not counting the E= and D= for the score. 

The decryption takes every n'th character wrapping around, where n is half the string length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so taking every nth character inverts taking every 2nd one.

If using a single function were allowed, I could do 44 bytes

def F(x,b):n=1+len(x)**b>>b;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True. The expression 1+len(x)**b>>b is equal to [2,len(x)/2+1][b].

Python 2, 61 bytes

E=lambda x:x[::2]+x[1::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

The first one encrypts, the second one decrypts. I'm not counting the E= and D= for the score. The idea of the decryption is to take every n'th character wrapping around, where n half the length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so multiplying the index by n reverses multiplying by 2.

If I were allowed to combine the two functions into one, I could do 45 bytes

def F(x,b):n=(len(x)/2)**b+1;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True.

Python 2, 61 bytes

E=lambda x:x[::2]+x[1::2]
D=lambda y:(-~len(y)/2*y)[::len(y)/2+1]

The first one encrypts, the second one decrypts. I'm not counting the E= and D= for the score. The idea of the decryption is to take every n'th character wrapping around, where n half the length round up. The reason this inverts is that 2 and n are inverses modulo the length of the string, so multiplying the index by n reverses multiplying by 2.

If I were allowed to combine the two functions into one, I could do 44 bytes

def F(x,b):n=1+len(x)**b>>b;return(n*x)[::n]

The encrypts when b is False and decrypts when b is True. The expression 1+len(x)**b>>b is equal to [2,len(x)/2+1][b].